Known liquid crystal electro-optical devices are TN type liquid crystal electro-optical devices using nematic liquid crystals, STN type liquid crystal electro-optical devices using nematic liquid crystals, ferroelectric liquid crystal electro-optical devices, and dispersion liquid crystal electro-optical devices. In a dispersion liquid crystal electro-optical device, a liquid crystal layer comprising a nematic, cholesteric, or smectic liquid crystal dispersed in a solid-phase polymer is sandwiched between a pair of substrates. The sandwiched liquid crystal is maintained in a particulate or spongy state. A known method of forming this liquid crystal layer consists in dispersing the liquid crystal in the polymer and shaping the polymer into a thin film on a substrate or a film. The liquid crystal is dispersed in the polymer by encapsulating the liquid crystal in a material. Materials which have been proposed as encapsulating materials include gelatin, gum arabic, and polyvinyl alcohol.
In these techniques, those liquid crystal molecules which are encapsulated in polyvinyl alcohol and exhibit a positive dielectric anisotropy in a thin film are lined up in the direction of the electric field in the presence of this field. If the liquid crystal is identical in refractive index to the polymer, transparency appears. On the other hand, where there exists no electric field, the molecules of the liquid crystal do not line up in a given direction; rather they are oriented randomly. Therefore, the liquid crystal differs in refractive index from the polymer. The incident light is scattered, and transmission of the light is hindered. In this state, the device appears white and cloudy. In this way, the encapsulated liquid crystal molecules are dispersed in a thin film polymer. Some structures are known other than the above structure. For example, in one structure, a liquid crystal material is dispersed in epoxy resin. Another structure utilizes phase separation between a liquid crystal and a photocurable substance. In a further structure, a liquid crystal is immersed in polymeric molecules which are bonded together in three dimensions. These liquid crystal electro-optical devices are herein collectively referred to as dispersion liquid crystal electro-optical devices.
One of the factors which determine the properties of a dispersion liquid crystal electro-optical device is the degree of dispersion in the region where the liquid crystal exists. In particular, in the structure comprising encapsulated liquid crystal molecules, the determining factor is the degree of dispersion of the capsules. In the structure using a polymer, the factor is the degree of dispersion of spatial portions. Where their uniformity is not good, the average electro-optical property of the liquid crystal material does not show steepness. Hence, the threshold value for activating the device is not fixed. A typical example of electro-optical properties of dispersion liquid crystals is shown in FIG. 14.
One method for solving this problem is to determine the threshold value, using thin film transistors (TFTs), called active devices. Also, this method is indispensable. Usually, n-channel TFTs are used in dispersion liquid crystal devices. Where only one kind of n-channel and p-channel TFTs is used to activate the device, it is impossible to sufficiently suppress the leakage current in off state. Therefore, it is necessary to form independent capacitive elements parallel with the capacitive components of the liquid-crystal element. FIG. 15 shows the structure of a typical liquid crystal device using TFTs of a single channel.
It is known that active liquid crystal electro-optical devices using TFTs have excellent performance. The TFTs are fabricated from an amorphous or polycrystalline semiconductor. One pixel is formed by a thin-film transistor of one conductivity type, i.e., either p- or n-type. Generally, an n-channel thin film transistor (NTFT) is connected in series with each pixel electrode. A typical example is shown in FIG. 5.
Generally, an active-matrix, liquid crystal electro-optical device has very numerous pixels, for example, 480.times.640 or 1260.times.960. To simplify the illustration, a matrix arrangement of 2.times.2 is shown in FIG. 5, where plural gate lines G.sub.1 and G.sub.2 and plural signal lines D.sub.1 and D.sub.2 are arranged so as to intersect each other. Pixel elements are installed at the intersections. Each pixel element comprises a liquid crystal portion 82 and a thin film transistor (TFT) portion 81. Signals are applied to the pixel elements from peripheral circuits 88 and 87 to selectively activate the pixels, for providing desired display.
However, where this liquid crystal electro-optical device is manufactured in practice and a display is provided, the output voltage V.sub.LC 80 from each TFT, or the input voltage to the liquid crystal, frequently does not go high when it should go high. Conversely, when it should go low, it often fails to go low. This is because the characteristic of each switching device applying a signal to a pixel electrode lacks symmetry. Specifically, the electrical characteristic of the charging to each pixel electrode is different from the electrical characteristic of the discharging from the electrode. Intrinsically, the liquid crystal 82 is insulative in operation. When the TFT is not conducting, the liquid-crystal potential V.sub.LC is in a floating condition. Since the liquid crystal 82 is equivalently a capacitor, the potential V.sub.LC is determined by the electric charge stored in it. Leakage of this electric charge takes place where the liquid crystal takes a comparatively small resistance of R.sub.LC, where dust or impurity ions exist, or where a resistance R.sub.GS 85 is formed because of pinholes in the gate insulating film of each TFT. Under this condition, the potential V.sub.LC does not take an intended value. Consequently, a high production yield cannot be achieved for a liquid crystal electro-optical device comprising one panel having 200 thousand to 5 million pixels.
Where a twisted nematic liquid crystal is used as the liquid crystal 82, a rubbed orientation film is formed on each electrode to orientate the liquid crystal. During the rubbing, static electricity is produced. This causes a weak dielectric breakdown. As a result, leakage takes place between adjacent pixels or conductors. Also, leakage occurs where the gate insulating film is weak.
It is quite important for an active liquid crystal electro-optical device that the liquid-crystal potential, or the input voltage to the liquid crystal, be maintained at the given initial value during one frame. In practice, however, active devices often malfunction. The practical situation is that the liquid-crystal potential is not always retained at the initial value during one frame.
When the liquid crystal is driven by supplying a signal thereto, if a large amount of one of positive voltage and negative voltage is applied to the liquid crystal as compared with the other, then electrolysis occurs, attacking or denaturing the liquid crystal material. In consequence, a satisfactory display cannot be provided. In order to solve such a problem, an AC signal is supplied. However, this AC signal is quite complex.